Building a Release for Windows 10 from Mac OS

I built a release version that could run on my Mac and sent it to a friend to try it out. He uses Windows 7. It works.

I sent it to another friend who uses Windows 10. The application doesn't work. The .app extension turns out into a folder on his screen (with contents similar to right-click->Show Package Contents for Mac).

I was wondering if this compatibility issue was due to the -spec flag on the qmake? I tried setting (in Additional arguments) -spec win32-clang-g++ and a bunch of other win32 mkspecs but, when building, I kept getting <QtCore/qchar.h> not found or unknown argument: -fno-keep-inline-dllexport. I attempted a thorough search across the web but failed to find a solution to be able to build.

So I thought maybe there was a special step for Windows 10? or maybe for Windows in general?

Alright...
Friend #1 used an application called WinZip and somehow unpacked the applications (which I sent in a .zip file which also contained a .db and a README.txt).

I told friend #2 to download WinZip. He did. Unzipped the folders. Tells me is still a bunch of folders.

So no indeed, my first friend didn't go into the folder to execute the application itself.

My second friend (the one using Windows 10) further informs me that the application inside the .app folder (what would be a Unix Executable on my Mac) has type File.
If he tries to rename the file to have a .exe extension, he gets a "This app can't run on your PC" error pop-up.

@ResistorInTheDark Somehow I don't really understand - "I built a release version that could run on my Mac and sent it to a friend to try it out. He uses Windows 7. It works.". How can a MacOS build of your app work on Windows?

To get a Windows build you will need to build on Windows. Either use a machine with Windows or a Virtual Machine.

@ResistorInTheDark
You're going to have a tough time setting up a cross compile chain from MacOS to Windows. It's possible, this StackOverflow thread may help you.
Besides the obvious hassels, you'll also have to crosscompile the whole Qt libabry, in the end, you're much better of by simply creating a vm and runnning windows & compiling of from that.

@ResistorInTheDark Why should your friend have windeployqt? It's your job to deploy the app for all platforms/OS you want to support.
So, again: you need Windows machine with a compiler (MinGW or VC++) and Qt. Then build your app there and use windeployqt to deploy your app and then send the result to your friend.
And I did NOT adviced that your friend needs to install Qt! You should read more careful what others write...